High temperature structural resins are needed which can be fabricated without the evolution of volatiles, which exhibit good mechanical performance under a variety of environmental conditions and are thereby useful for structural applications in the construction of advanced aircraft and space vehicles. Although a significant effort has been devoted to the development of high temperature structural resins, presently available materials are plagued by their own unique combination of problems such as poor processability, solvent and moisture sensivity, microcracking, low impact strength, and poor dimensional stability at elevated temperature. In an attempt to develop a more aceptable structural resin, attention herein has focused on the use of the ethynyl and phenyethynyl groups as a means of rigidizing and crosslinking polymers.
Polyphenylquinoxalines (PPQ) are high temperature thermoplastics which exhibited excellent performance as structural resins (e.g. adhesives and composite matrices) at temperatures less than their glass transition or heat distortion temperatures. Polyphenylquinoxalines have also exhibited good potential for use as protective coatings even though they exhibit a sensitivity toward certain solvents. It is highly desirable to increase their use temperature and improve their solvent resistance.
Several routes have been used in an attempt to improve the dimensional stability of these polymers at elevated temperatures. Linear thermoplastic PPQs have been exposed (post cured) to high temperatures to induce crosslinking. This resulted in an increase in the use temperature of the polymers but at the sacrifice of lowering room temperature properties and the use life (long term stability) at elevated temperature (See P. M. Hergenrother, Polymer Engineering and Science 16(5), 303 (1976)). Latent crosslinking groups such as cyanato and cyano have been incorporated in PPQs in an attempt to obtain thermally induced crosslinking to reduce the high temperature thermoplasticity (See P. M. Hergenrother, Macromolecules, Vol. 7, p. 575 (1974) and P. M. Hergenrother, U.S. Pat. No. 3,852,243 (1974) to the Boeing Company). The PPQs containing cyanato groups were not processable due to the relatively low temperature reaction of these groups which resulted in inhibition of polymer flow. Those containing the cyano group required extremely high temperatures (e.g. more than 400.degree. C.) to effect moderate crosslinking. A trifunctional momoner (a tris(phenyl-.alpha.-diketone) was previously used to prepare highly crosslinked PPQs, see R. T. Rafter and E. S. Harrison, ACS Div. Org. Coatings and Plastics Chem. Prep., 35(2), 204 (1975)). These materials were extremely difficult to process as adhesives or composite matrices, requiring high pressures and temperatures. Extremely rigid PPQs with high T.sub.g s were prepared using aromatic fused ring tetraamines and aromatic bis(.alpha.-diketones), see F. L. Hedberg and F. E. Arnold, J. Polymer Sci., Polymer Chem. Ed. 14, 2607 (1976). Ether-ketone-sulfone polymers containing pendant ethynyl groups capable of undergoing a thermally induced crosslinking reaction have been reported, see C. Samyn and C. S. Marvel, J. Polymer Sci. Polymer Chem. Ed. 13, 1095 (1975). These polymers were prepared by a route which is entirely different from the present invention. A precursor linear ether-ketone-sulfone polymer containing pendant acetyl groups was subsequently converted to ethynyl groups via the Vilsmeyer reaction. This reaction is known not to occur quantitatively (incomplete conversion and by-product formation). As a result, the crosslinked polymers exhibited poorer thermooxidative stability than anticipated. In an attempt to increase the use temperature (T.sub.g) of PPQs, 2,2'-di(phenylethynyl)biphenyl moieties were incorporated within the backbone of the polymer chain. The polymer was subsequently thermally reacted to form rigid 9-phenyldibenz(a,c)-anthracene units, see F. L. Hedberg and F. E. Arnold, J. Polymer Sci. Polymer Chem. Ed. 14, 2606 (1976); and F. L. Hedberg and F. E. Arnold, U.S. Pat. No. 3,876,614 (1975) to U.S. Air Force. This work involves intramolecular reaction to rigidize the polymer and thereby differs substantially from the present invention which involves intermolecular reaction crosslinking. The monomer containing the 2,2'-di(phenylethynyl)biphenyl moiety is made by a very difficult multistep route which is economically impractical. Due to the synthetic difficulties, the final polymers have not been thoroughly evaluated. Ethynyl terminated aromatic polyphenylquinoxaline compositions, (or ethynyl end-capped quinoxaline oligomers) which cure by addition reactions are also disclosed in U.S. Pat. No. 3,966,729 to Kovar et al.
Each of these prior art compositions and the various routes employed therein to increase the dimensional stability of PPQs resulted in one or more of the following disadvantages; poor processability, lower long term stability at elevated temperatures, expensive multistep synthetic routes to the monomers, high temperature required for crosslinking, and inability to control the degree of crosslinking.
It is therefore an object of the present invention to provide a new resin composition produced from the polyphenylquinoxalines and containing various amounts of latent crosslinking groups to provide crosslinking.
It is another object of the present invention to provide novel crosslinking resin compositions having improved elevated temperature use capabilities.
A further object of the present invention is a process for improving the use temperature and solvent resistance of selected thermoplastic resins by incorporating in the structure thereof crosslinking groups.
An additional object of the present invention is a process for improving the physical property use characteristics of the polyphenylquinoxaline resins by adding pendant ethynyl or phenylethynyl crosslinking groups thereto.
Other objects and advantages of the present invention will be more readily apparent to those skilled in the art as the same becomes better understood with reference to the following description and specific examples.